/*
 * Licensed to Crate under one or more contributor license agreements.
 * See the NOTICE file distributed with this work for additional
 * information regarding copyright ownership.  Crate licenses this file
 * to you under the Apache License, Version 2.0 (the "License"); you may
 * not use this file except in compliance with the License.  You may
 * obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
 * implied.  See the License for the specific language governing
 * permissions and limitations under the License.
 *
 * However, if you have executed another commercial license agreement
 * with Crate these terms will supersede the license and you may use the
 * software solely pursuant to the terms of the relevant commercial
 * agreement.
 */

package io.crate.blob;

import org.elasticsearch.cluster.routing.Murmur3HashFunction;

import java.util.concurrent.Semaphore;

/**
 * Offers coordination support for blob operations which need to execute in a mutually exclusive way
 * (eg. blob commits for the same blob digest cannot execute concurrently)
 *
 * Should be used like this:
 *
 * <pre>
 *    Semaphore semaphore = blobCoordinator.digestCoordinator(digest);
 *    try {
 *       semaphore.acquire();
 *
 *       try {
 *           // guaranteed single thread access to execute operation
 *       } finally {
 *           semaphore.release();
 *       }
 *    } catch(InterruptedException e) {
 *        // couldn't acquire semaphore
 *    }
 * </pre>
 */
class BlobCoordinator {

    private final Semaphore[] digestBinarySemaphores;

    BlobCoordinator() {
        digestBinarySemaphores = new Semaphore[500];
        for (int i = 0; i < digestBinarySemaphores.length; i++) {
            digestBinarySemaphores[i] = new Semaphore(1);
        }
    }

    Semaphore digestCoordinator(String digest) {
        return digestBinarySemaphores[Math.abs(Murmur3HashFunction.hash(digest) % digestBinarySemaphores.length)];
    }
}
